It is well known that the physical properties of various organic polymers deteriorate and color changes take place during processing of the polymer and during exposure of formed polymer products to certain environments. The prime examples of polymers which are susceptible to degradation during processing are the halogen-containing polymers such as the vinyl and vinylidene polymers in which the halogen is attached directly to carbon atoms. Poly (vinyl chloride) or PVC, copolymers of vinyl chloride and vinyl acetate, and poly (vinylidene chloride), the principal resin in self-clinging transparent food wraps, are the most familiar polymers which require stabilization for their survival during fabrication into pipes, window casings, siding, bottles, wall covering, packaging film, and the like. When such polymers are processed at elevated temperatures, undesirable color changes often occur within the first 5 to 10 minutes as well as during later stages of the processing. Haziness, which sometimes accompanies the color changes, is particularly undesirable where clear products are needed.
The addition of heat stabilizers to such polymers has been absolutely essential to the wide-spread utility of the polymers. From a great deal of work in the development of more and more effective heat stabilizers there has emerged two principal classes: organotin compounds and mixed metal combinations. Organotin-based heat stabilizers are the most efficient and widely used stabilizers for rigid PVC. Synergistic combinations of alkyltin mercaptides and free mercaptans are particularly efficient heat stabilizers for rigid PVC during extrusion. They have not been entirely satisfactory, however, because of several failings on the part of the mercaptan synergist and are not used in flexible PVC. Many mercaptans give off an offensive odor even at room temperature and the odor grows worse at PVC processing temperatures. The oxidative stability of the mercaptans is very often very poor. Oxidation of the free mercaptans diminishes the synergism. Thus, a combination having an enhanced synergism would be welcomed by the PVC industry. Also, because of the end-use of articles made from some polymers, many polymeric compositions require the presence of both biocides and heat stabilizers but the use of the organotin mercaptide/mercaptan combination in such a composition is often frustrated by the tendency of the free mercaptan to deactivate a biocide such as the much used OBPA (10, 10'-oxybisphenoxarsine).
Another shortcoming of such mercaptan-containing heat stabilizers is that they fail to protect the polymer compositions against the degrading effects of ultraviolet light and weathering. Upon prolonged exposure to ambient light and outdoor weathering conditions, products formed from these so-stabilized polymers readily tend to discolor and decompose with attendant loss in physical properties, e.g., tensile strength, flexibility, and impact resistance, which shortens the useful life of such articles.
With increased use of halogen-containing polymer compositions to form products designed for exterior weatherable applications, for example, vinyl house siding, window frames, window profiles, corrugated roofing, door stripping, etc., it is increasingly important to develop a stabilizer that will not only protect the polymers against heat encountered during processing and fabrication, but also protect the polymers against the long term effects of ambient light and outdoor weathering. Such a stabilizer is particularly desired for clear products which have very poor light stability and weatherability, as they generally do not contain opaque fillers and pigments, such as titanium dioxide, which inhibit light from penetrating therein and thus reduce the rate of polymer degradation.
In U.S. Pat. No. 3,660,331, Ludwig teaches the stabilization of vinyl halide resins by certain thioethers and thioesters of tetrahydropyran. Better heat stabilizer compositions are still needed, however. Heat stabilizers that offer improved ultraviolet light stability and weatherablity to halogen-containing polymer compositions, particularly clear polymers, are even further needed. The thioethers/low level phenyl salicylate stabilizer combinations of this invention satisfy such needs.